In general the compression molding process for making recorded discs or records has proceeded along the following general lines: the molding composition is first thoroughly mixed and blended in a blade type mixer, as for example a Banbury mixer, from which it is fed into an extruder of the screw type. The extruder functions to mix and fuse the molding composition of plastic resinous material supplied thereto; and, thereafter, discharge the composition at a desired molding temperature in metered quantities for transport to the record or disc molding press. More particularly, a measured quantity or charge of the fused and heated molding material is caused to issue from the nozzle of the extruder into record molding apparatus, including means for receiving, forming, and holding the charge of moldable material, which means is often referred to as a "shot cup" or "forming cup". The measured quantity of molding composition which is formed and shaped by the shot cup is generally referred to as a "shot" or "preform". The forming cup is often pivotally mounted on a frame closely adjacent the extruder nozzle and is usually also adjacent to the molding press. This preform cup or mold may be so arranged as to transport the shot or preform to the molding press portion of the record molding apparatus, and thereafter deposit the charge at the proper point in the press operation cycle between the platens or pieces of the mold of the record press.
After the shot or preform has been formed, the same is then centrally placed, either automatically or manually by operator, between a pair of heated molds or press platens and an air valve is activated which closes the press platens and starts the operation of a timer controlled cycle of heating and cooling operations within the platens. The molds are closed against the preform, meeting the thermoplastic and compressing it, and causing it to flow outwardly to fill the mold cavity and define the record contour. At the end of the cycle, the press opens automatically and the disc is removed, either automatically or manually by an operator, from between the platens. At this point, the record will normally still have a ring of flash material attached to its outer edge, and this has to be removed either by hand operation or trimmed in a trimming apparatus.
In the molding of the record or disc, it is highly desirable to prevent, or at least to minimize, surface defects, such as voids, which can disrupt the grooves in the surface of the disc. In the making of high density information discs, such as video discs, the prevention of such surface defects is even more highly desirable. These discs utilize groove packing densitites on the order of 10,000 grooves per inch (3,937 per centimeter); therefore, even the smallest surface defect can disrupt a large number of the grooves.
As previously explained, the preforms from which the video discs are molded are generally formed at the extruder, and closely adjacent the molding press, in a two-part, cup-shaped mold which is generally referred to as the "shot cup" or "preform cup". The heated molding composition is injected into the preform mold through an opening therein until the cup is filled with the moldable material; forming the preform, which is then removed from the cup and placed in the record press. When the heated molding material is injected into the cup, it extrudes in a snake-like fashion across the interior of the cup until the same is filled. In this process, the molding material comes in contact with the relatively cool interior of the cup to result in the formation of a skin-like outer layer on the preform. This skin, along with other non-uniformities across the plastic material which may set-up during the uneven cooling thereof, can result in inconsistencies and variations in the density of the preform. These inconsistencies and variations adversely affect the quality of, and can result in defects in, the recorded disc made from the preform.